1. Field of the Invention
This invention relates to a control system for an automatic vehicle transmission, particularly to a control system for oscillating a linear solenoid (more specifically, an actuator such as a pressure control valve, etc., connected or associated to linear solenoid) to remove foreign matter such as grit from the valve or actuator.
2. Description of the Related Art
Automatic vehicle transmissions are commonly shifted by using linear solenoid valves or the like (actuators) to operate hydraulic clutches. A technology for this is taught, for example, by Japanese Patent Publication No. Hei 5(1993)-17,430. Upon being supplied with current, the linear solenoid (more precisely, a pressure control valve associated with the linear solenoid) supplies the hydraulic clutch with hydraulic pressure proportional to the supplied current so as to engage the hydraulic clutch and shift gears.
Nowadays, direct control using many actuators such as linear solenoid valves, i.e., control without use of an accumulator or other such hydraulic pressure storage means, is becoming increasingly common in hydraulically operated automatic vehicle transmissions.
One problem encountered by hydraulically operated automatic vehicle transmissions is grit (foreign particles) getting mixed into the oil (automatic transmission fluid or ATF). Although most of the grit is caught by a magnet, filter and the like installed in the hydraulic control circuit, total removal of grit of a grain diameter under 100 .mu.m is difficult. The remaining grit finds its way into the linear solenoids, pressure control valves and other components, where it is liable to cause locking, abnormal stroke and other problems.
Japanese Laid-Open Patent Application No. Hei 4(1992)-248,059, for example, teaches a method for removing grit by supplying a linear solenoid with dither current, i.e., oscillation-exciting current of a prescribed frequency and amplitude to vibrate the valve. The supplied dither current should be of large amplitude to ensure reliable grit removal.
However, supply of high-amplitude dither current results in large movement of the solenoid plunger and the pressure control valve. The large fluctuation in the hydraulic control pressure thus produces increased gear-shift shock and causes clutch oscillation or vibration. If the amplitude of the dither current is made small to avoid these problems, thorough grit removal cannot be achieved.